In 1955 the NS switched to a new signaling system. The new 1955 signaling system gave an
almost complete de-coupling of speed and clearance signalling. The base signal pole has
three lights (In the beginning there was a fourth, yellow, light but that light has been abandoned.)
The three basic lights have different colors, green, yellow and red.
Additionally, a matrix board board with a number for extra speed signalling can also be shown.
The three lights control the clearance of the track ahead of the train. Only one light can be shown at a time.
The yellow light gives a speed signal, at first 30km/h, later 40km/h. Green allows passing and red orders stop.
If a specific speed has to be forced upon the train, a matrix board can be shown.

green

green flash with number 7

green flash

passing permitted

passing permitted with 70 km/h

passing permitted with 40 km/h

Preparations to make the system fail-safe :

The lights are fitted with a long "sun cap" above each light to prevent sunlight
from falling on the mirror and shining back to the driver.

The red light is always at the the bottom. With heavy snow it is believed that
snow can stay on top of the "sun cap" and thus prevent a light from being seen.
When red is at the bottom, no snow lying on a "sun cap" can obscure this light.

The matrix board have more reflectors in it than necessary to show a number. If
sunlight reflects on the mirrors, a nonsense figure would be displayed.

yellow

yellow flash

reduce speed to 40km/h the next signal can show a stop sign.

Ride on sight, reduce speed (max 40km/h) so that the train can stop before any obstackel.

Why is the green light flashing when a number is shown and yellow not? Fail-safe! Green flash is a more restrictive
signal than green flash 6.
So when in case of a failure the number is not shown, yellow 6 will turn into yellow, being a more restrictive signal.
Green 6 would turn to green and the driver would think that track speed is allowed, which is not. Therefore the flash
is introduced.

yellow with number 6

yellow with flashing number 8

red

reduce speed to 60 km/h. This speed has to be reached before the next signal and the next signal will not show stop

reduce speed to 80 km/h. This speed will not be reached before the next signal but braking has to continue until the desired speed is reached